The stated goal of this research is to quantify trunk stability and associated spinal mechanics in order to identify and control occupational hazards for LBD risk. A major limitation in controlling the prevalence of occupational low-back disorders (LBDs) is the inability to quantify trunk stability or explain the biomechanical injury mechanism. LBD are common in occupations with high exposure to unstable events and sudden loads such as construction and nursing. In general, lifts performed from twisted, laterally bent or dynamic postures are considered significant LBD risk factors. The cause of this increased risk may be associated with the reduced trunk stability in these MMH postures. Thus, occupational requirements for musculoskeletal stability may be a significant factor influencing the prevalence LBD and controlling the occupationally-related risk. Three specific aims will be accomplished: (1) Develop a biomechanical model of in vivo trunk/spinal stability; (2) Assess the influence of MMH task parameters upon the relative stability of the trunk; and (3) Quantify spinal loads and musculoskeletal behavior in response to unstable events. Research from the investigator's group has demonstrated that occupational LBD risk is strongly influence by trunk motion dynamics. Electromyographic (EMG) measurements suggest that lifting dynamics increases muscle co-activity, caused by the need for enhanced trunk stability. Scientific literature suggests the level of stability will influence the injury tolerance of the spine. Furthermore, the investigator's work shows that when unstable events or sudden loads occur, the musculoskeletal system over-reacts, loading the spine approximately 3 times greater than an equivalent stable exertion. The EMG-assisted biomechanical model employed to achieve these results has demonstrated that muscle co-contraction significantly increases spinal load, particularly during dynamic, twisting and lateral exertions typical of MMH tasks. Since trunk stability is a function of muscle co-activity, this EMG-assisted model is ideal for adaptation to stability analyses. Laboratory facilities can be employed to develop and validate the stability model as well as measuring trunk stability during a variety of MMH tasks. The candidate directed much of the EMG-assisted model development and validation. With this proposal, it is his intention to re-focus his research toward the assessment of MMH task stability and its interaction with spinal load. This will be done to identify the cause of occupationally-related LBD, develop a method to identify hazards for LBD risk in the work place, and control the prevalence of occupationally related LBD.